1. Field of the Invention
The present invention relates to a high-pressure discharge lamp.
2. Description of the Related Art
Extra-high-pressure mercury lamps are currently being used as the light source of liquid crystal projectors.
Compared to such as a metal halide lamp, a typical mercury lamp has weak light emission in the red region in the optical color rendering (spectrum distribution). Increasing the operating pressure (the internal pressure of the lamp during illumination), however, allows a continuous spectrum to be obtained in the red region even with the mercury lamp, and further, produces a light source that is superior from the viewpoints of both efficiency characteristics and life expectancy characteristics.
A high-pressure discharge lamp includes bulb 1 that is composed of: a spherical portion that forms discharge chamber 1a in the center of a glass tube; and slender glass sealing sections 1b and 1b′ for sealing the openings at the two ends of the glass tube, as shown in FIG. 1. In discharge chamber 1a, a pair of electrodes 4 and 4′ provided with cooling coils 2 and 2′ are arranged such that the tips of the electrodes 4 and ′4 confront each other. The back ends of these electrodes 4 and 4′ are connected to lead rods 7 and 7′, respectively, with molybdenum foil parts (Mo foil) 6 and 6′, respectively, interposed. The back ends of electrodes 4 and 4′, the molybdenum foil parts 6, and 6″, and one end of each of lead rods 7 and 7′ are hermetically buried within the glass that forms glass sealing section 1b and lb′. Finally, mercury, halogen gas, and an inert gas are sealed inside discharge chamber 1a. 
However, since the operating pressure of the extra-high-pressure mercury lamp that is receiving attention as the light source of a liquid crystal projector is 200 atmospheres or more, a major problem is the prevention of damage to the lamp itself. In particular, rupture of the lamp produces a loud noise and scatters harmful substances such as mercury and halogen gas and thus poses a danger to the end user, and various measures for preventing breakage have therefore been proposed. For example, Japanese Patent Laid-Open No. 111226/1999 proposes that metal foil parts (for example molybdenum foil parts) be bonded to the electrodes that are positioned in the discharge space and that these metal foil parts be embedded in the glass that forms the sealing sections at the two ends of the lamp, the electrode side of these metal foil parts being formed in a rounded shape (curved shape).
In this official gazette, the lack of angular portions in the electrode-side ends of the metal foil parts inside the glass sealing sections provides a suppression of both concentrations of stress against these electrode-side ends and the occurrence of cracks in the electrode-side ends of the metal foil parts, whereby sufficient pressure resistance for the operating pressure can be obtained at both ends of the swelled glass portion.
Alternatively, Japanese Patent Laid-Open No. 250504/2001 proposes a construction in which the ends of electrodes and metal foil parts that are welded to these electrodes are sealed inside the sealing sections that seal the openings at the two ends of a glass tube, the welded portions of the electrodes and the metal foil parts being further covered by metal foil parts such that the ends of the electrodes are not exposed, and further, the width of the electrode-side ends of the metal foil parts being less than the width of the opposite-side ends of the electrodes. In particular, the metal foil parts are provided with a triangular shape, and the edge portions of the metal foil parts of this triangular shape are streamlined.
In this official gazette, the lack of any stepped portions between the electrodes and the metal foil parts at the welded portions of the electrodes and metal foil parts and the lack of any angles in the electrode-side ends of the metal foil parts enable a reduction of cracks that occur in the glass in the vicinity of the welded portions of the electrodes and metal foil parts when melting the two ends of the glass tube to form sealing sections, thereby obtaining an improvement in the pressure resistance of the lamp.
Japanese Patent No. 3204189 further proposes a construction in which metal foil parts (for example, molybdenum foil parts) that are bonded to electrodes that are positioned in the discharge space are buried inside the glass that forms the sealing sections at the two ends of the lamp, and further, in which coils are wrapped around the portions of the electrodes that are buried in the sealing sections.
In this official gazette, the interposition of coils between the electrodes and glass enables a reduction of the occurrence of cracks in the glass that contacts the electrode surfaces during the process of forming the sealing sections. In addition, the patent further reports that the ability to form the sealing sections at high temperature enables an improvement of the close contact between the metal foil parts and glass, whereby a lamp having sufficient pressure resistance can be provided.
Nevertheless, the measures for preventing breakage according to Japanese Patent Laid-Open No. 111226/1999 and Japanese Patent Laid-Open No. 250504/2001 focus only on the concentration of stress against the electrode-side ends of the metal foil parts inside the glass that is formed at the sealing sections, and further, the concentration of stress against the ends of the electrodes on the side of the metal foil parts. The measure for preventing damage according to Japanese Patent No. 3204189 focuses on the occurrence of cracks in the glass that contacts the electrode surface during the process of forming the sealing sections as well as on the close contact between the glass and the metal foil parts.
The primary causes for the occurrence of breakage of the lamp itself include a variety of causes in addition to those described in each of the above-described official gazettes, i.e., glass cracks that are caused by the difference in thermal expansion between the electrodes and the glass that is in contact with the electrodes during cooling following formation of the sealing sections, glass cracks that are caused by the concentration of stress against the ends of the electrodes, and glass cracks that are caused by the concentration of stress against the ends of the metal foil parts; and may also include a combination of these causes. As a consequence, the implementation of one or two of the countermeasures described in each of the official gazettes cannot be expected to have an actual effect.
Furthermore, another factor in addition to the factors described in each of the above-described official gazettes is the occurrence of a gap between the glass and the portions of the electrodes that are embedded in the glass. When such a gap is present, the high pressure that is produced inside the lamp upon lighting causes halogen gas to pass through the gap between the electrodes and the glass and bring about corrosion of the junction between the electrodes and the metal foil as well as corrosion of the metal foil, and this corrosion eventually leads to rupture of the lamp.
In the construction in which coils are wrapped around the portions of the electrodes that are embedded in the glass, as well, when an absolutely hermetic seal is not achieved between the electrodes and the coils, gaps occur between the glass and the portions of the electrodes that are embedded in the glass, and halogen gas that infiltrates this gap passes between the electrodes and the coils and brings about the above-described corrosion that leads to rupture of the lamp. Japanese Patent No. 3204189 discloses a construction in which coils are embedded only in the glass and are not exposed in the light emission space but discloses nothing regarding corrosion caused by halogen gas to the junction portion of the electrodes and metal foil as well as the metal foil itself.
In a construction in which coils are wound around the portions of the electrodes that are sealed inside the sealing sections, deformation of the metal foil that occurs when winding the coils is also a factor for shortening the life expectancy of the lamp. In other words, deformation of the metal foil reduces the close contact between the glass and metal foil, causing separation of the glass and metal foil and bringing about gas leakage of the discharge space.